Power supply rejection for pulse width modulated amplifiers and automatic gain control

ABSTRACT

A circuit according to the present invention improves the power supply rejection ratio for a pulse width modulated digital amplifier and can be used as a compressor and/or limiter, either in conjunction with an amplifier or independently. The circuit preferably operates by using voltage level translation to vary the amplitude of a triangle wave in response to changes in power supply voltage prior to input of the wave into a comparator of the PWM device. Because the circuit operates to improve power supply rejection, little or no distortion is introduced into the signal when used as a compressor and/or limiter. Additionally, the circuit is optionally implemented in a Class D amplifier, and provides a lower cost of implementation than conventional designs in such implementations.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/030,318 filed Jan. 7, 2005, now U.S. Pat. No. 7,135,922 which claimspriority to U.S. Provisional Patent Application Ser. No. 60/535,134,filed Jan. 7, 2004, both of which applications are incorporated hereinby reference in their entirety.

This application includes material which is subject to copyrightprotection. The copyright owner has no objection to the facsimilereproduction by anyone of the patent disclosure, as it appears in thePatent and Trademark Office files or records, but otherwise reserves allcopyright rights whatsoever.

BACKGROUND OF THE INVENTION

Class D amplifiers differ from conventional power amplifiers, such asClass A and Class B power amplifiers, in various respects. Conventionalpower amplifiers have an output voltage or current that isproportionally larger than its input. The output devices operate in thelinear region, where the device is partially “on”. As a result, much ofthe power that is supplied to the amplifier converts to heat and is notefficiently utilized. Larger power transformers are needed as well aslarger heat sinks, which remove heat from the output devices.

Unlike the partially “on”power amplifiers, Class D amplifiers operatesuch that the output devices are either on or off. Because the devicesare on or off, the devices convert less power to heat. As a result, asmaller heat sink and power transformer can be used. However, there aresome limitations associated with Class D amplifiers.

Due to power supply rejection, Class D amplifiers may suffer fromreduced dynamic range and operate non-linearly. The power supplyrejection ratio (“PSRR”) is the ratio of the change in output voltage ofan amplifier to the change in power supply voltage, and can be expressedas 20*log (Voltage output change/Voltage supply change). In linear powersupplies with a power transformer, as more current is consumed by theamplifier, the voltage provided by the power transformer is reduced.Class D amplifiers typically have a 0 dB PSRR. Any change in the powersupply voltage is reflected directly at the output of the amplifier. Ifthe power supply voltage is reduced by −6 dB for example, then theoutput signal will be reduced by −6 dB as well, reducing the dynamicrange and causing non-linear operation. Dynamic range is the audio rangefrom the lowest to the highest detectable volume signal output by theamplifier. If the reduction in the power rails reduces the highest levelof amplifier output and maintains the level of the lowest signal, thenthe dynamic range is effectively reduced. As a result, the listener doesnot hear the louder passages as loud as intended, while the softerpassages are produced as intended.

Audio power amplifiers are typically designed to drive loudspeakers. Insubwoofer applications, it may be desirable to reduce the dynamic rangeunder very controlled conditions using a compressor or limiter, whichare types of automatic gain control circuits. A compressor changes thegain of an amplifier based on signal level. For example, an input of +6dB into a compressor may result in an output of +3 dB when the signal isabove a predetermined threshold. A limiter does not change the outputwith an increase of input signal, when the signal is above a setthreshold. For example, an input of +6 dB into a limiter may result inno increase of signal on the output. Limiters and compressors can beexpensive to implement. Most low cost compressor or limiter designsintroduce harmonic distortion into the signal.

A conventional Class D audio amplifier as described in Motorolaapplication note AN1042, “High Fidelity Switching Audio Amplifiers UsingTMOS Power MOSFETs” by Donald E. Pauly, which is hereby incorporated byreference. The design for power supply correction implements costlytransformers. Additionally, the Audio Engineering Society preprintnumbers 4446 and 4673 discuss error correction using feedback techniquesand patented feed forward correction. However, feedback alone does notgive enough correction for power supply errors.

Pulse Width Modulation is a well-known technique for synthesizing atrain of pulses in pulse-modulation-based power amplifiers. Severalexamples of pulse width modulation techniques are detailed in “A Reviewand Comparison of Pulse Width Modulation (PWM) Methods For Analog andDigital Input Switching Power Amplifiers” by Karsten Nielsen, which isincorporated herein by reference in its entirety. Further discussion ofPWM can be found in “Comparing Nonlinear With Linear Control Methods forError Correction in Switching Audio Amplifier Output Stages,” by ThomasTaul, Karsten Nielsen, and Michael A. E. Andersen, which is herebyincorporated by reference. In one embodiment, the pulse width modulateddigital amplifier may be a Class D amplifier.

SUMMARY OF THE INVENTION

A circuit according to the present invention improves the power supplyrejection ratio for a pulse width modulated digital amplifier and can beused as a compressor and/or limiter, whether simultaneously orindependently. The circuit preferably operates by using voltage leveltranslation to vary the amplitude of a triangle wave in response tochanges in power supply voltage prior to the input of the wave into acomparator of the PWM device. Because the circuit provides improvedpower supply rejection, little or no distortion is introduced into thesignal when used as a compressor and/or limiter. The circuit may beimplemented in a Class D amplifier. The cost of implementation is lowerthan conventional designs when used in conjunction with a Class Damplifier. Furthermore, the circuit is more efficient than conventionaldesigns, so the power supply and transistor heat sinks can be smallerthan in those conventional designs.

In one embodiment, a circuit for providing improved power supplyrejection of a pulse width modulated digital amplifier includes abuffering mechanism that buffers divided reference voltages; anoscillator which outputs a first waveform, which can be combined withthe divided reference voltages; a subcircuit that performs a voltagelevel translation, wherein the voltage level translation outputs asecond waveform having a similar frequency to the first waveform and hasan amplitude that varies between the divided reference voltages; and anintegrator that converts the second waveform into a triangle waveformthat can be used with the pulse width modulated digital amplifier. Thepulse-width-modulated digital amplifier may be a Class D amplifier. Thebuffering mechanism is preferably an operational amplifier. Thebuffering mechanism ensures an accurate low-impedance voltage. Thesecond waveform is a square waveform. The oscillator is a fixedoscillator. The first waveform has a 50% duty cycle. The dividedreference voltages can include ground.

In another embodiment, a circuit useful as a compressor or limiter witha Class D amplifier comprises means for rectifying an audio inputsignal; means for DC amplifying the audio input signal; means forinjecting the audio input signal into a circuit for providing improvedpower supply rejection, the circuit comprising a buffering mechanismthat buffers divided reference voltages; an oscillator which outputs afirst waveform, wherein the first waveform can be combined with thedivided reference voltages; a subcircuit that performs a voltage leveltranslation, wherein the voltage level translation outputs a secondwaveform having a similar frequency to the first waveform, wherein thesecond waveform has an amplitude that varies between the dividedreference voltages; and an integrator that converts the second waveforminto a triangle waveform that can be used with the pulse width modulateddigital amplifier.

In yet another embodiment, a circuit useful as a compressor or limitercomprises means for rectifying an audio input signal; means for DCamplifying the audio input signal; means for injecting the audio inputsignal into a circuit for providing improved power supply rejection, thecircuit comprising a buffering mechanism that buffers divided referencevoltages; an oscillator which outputs a first waveform, wherein thefirst waveform can be combined with the divided reference voltages; asubcircuit that performs a voltage level translation, wherein thevoltage level translation outputs a second waveform having a similarfrequency to the first waveform, wherein the second waveform has anamplitude that varies between the divided reference voltages; anintegrator that converts the second waveform into a triangle waveformthat can be used with the pulse width modulated digital amplifier; meansfor inputting pulse-width-modulated signals to a differential amplifier;and a lowpass filter to restore audio to the output.

The foregoing and other objects, features, and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments as illustrated in the accompanyingdrawings, in which reference characters refer to the same partsthroughout the various views. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of atleast one embodiment of the invention.

In the drawings:

FIG. 1 illustrates the basic operation of a Class D amplifier.

FIG. 2 is a circuit in an amplifier used for power supply rejectionaccording to an embodiment of the present invention.

FIG. 3 is a circuit in an amplifier using only a positive voltage railaccording to an embodiment of the present invention.

FIG. 4 is a circuit having a compressor according to an embodiment ofthe present invention.

FIG. 5 is a circuit having a compressor in an amplifier that uses only apositive voltage rail according to an embodiment of the presentinvention.

FIG. 6 is a circuit having a compressor in an amplifier according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

With reference to FIG. 1, in the basic operation of a class D amplifier,an audio input signal (A) goes into a first comparator (C). The audioinput signal is also inverted and then goes into a second comparator(D). A triangle wave (B) is input into both comparators. Within bothcomparators (C, D), the audio input signal is compared to the trianglewave. If the audio input signal is higher than the triangle wave, thenthe comparator output is high. If the audio input signal is lower thanthe triangle wave, then the comparator output is low. The comparatorsoutput pulse width modulated (“PWM”) signals (PWM A, PWM B) thatcorrelate to the audio input signal. The PWM signal is rectified in thepower Metal-Oxide Semiconductor Field-Effect Transistors (“MOSFETs”),which have a PWM output that switches between the +/−voltage rails (E,F). First and second LC low pass filters (G, H) remove most of theswitching signal and convert it back to audio. Since the audio isinverted before the second comparator (D), the second filter (H) outputhas an inverted signal as compared to the first filter (G) output. Ifthe amplitude of the triangle wave is reduced by an amount, then thegain of the amplifier is increased by that same amount. For example, ifthe triangle wave is decreased by −6 dB, the amplifier gain is increasedby +6 dB. Because the power MOSFETs switch between the +/−voltagesupplies, a decrease of power supply voltage by −6 dB will decrease theaudio output by −6 dB.

A desirable PSRR is obtained by compensating for a decrease of gain dueto changes in power supply voltage by an equivalent increase of gain dueto triangle wave amplitude reduction. In other words, if the amplifieroutput level is reduced because the power supply voltage is reduced,then the triangle wave level is reduced by the amount that the powersupply voltage is reduced. The reduction of power supply causes thesquare wave to reduce its output because that output is set to be aratio of the power supply voltage. The square wave is then convertedinto a triangle wave. In one embodiment, this approach allows more thanone triangle wave to be generated by one square wave. Desirable PSRR isachieved because the amount of decrease in amplitude from the powersupply voltage reduction equals the amount of increase caused by thetriangle wave reduction.

FIG. 2 illustrates a circuit that can be used for power supply rejectiononly, when the power supply has +/−DC voltages. The circuit connects tothe unregulated power supply of the amplifier at points B, C and allop-amps use a regulated +/−15V. R1 and R2 divide the unregulatedpositive voltage and buffer the voltage through U1A (D). R8 and R10divide the unregulated negative voltage and buffer the voltage throughU1B (E). The oscillator outputs a square wave (A) and injects the signalinto MOSFETs Q1 and Q2. The output of Q1 and Q2 (F) is a square wave atthe same frequency of the oscillator and having an amplitude of thevoltage difference between the outputs of U1A and U1B (D, E). The squarewave (F) is capacitively coupled through C5 to the input of U2A. C2 andR5 create a low pass filter that converts the square wave (F) to atriangle wave (G). The amplitude of the triangle wave changesproportionally to change in the +/−unregulated voltage (B, C). Thesquare wave output (F) may be input to multiple integrators (U2A) inmulti-channel systems, where different mean amplitudes of the trianglewave are desired and all output stages are connected to the same voltagerails.

In applications where cost is a concern and performance may be degradedslightly, C4, R3, R6, D1, Q1 may be removed and R4 may be added. Thevalue of R4 affects the quality of the square wave from the output (F),which in turn affects the quality of the triangle wave. For improvedperformance U2A may be replaced by a high speed op-amp such as LM6172.

FIG. 3 illustrates an embodiment wherein an amplifier design uses only apositive voltage rail. In applications where cost is a concern andperformance may be degraded slightly, C4, C6, R3, R6, R9, R11, D1, D2,Q1 may be removed and R4 may be added. Additionally, the gate of Q2would be connected directly to oscillator output. The value of R4effects the quality of the square wave from the output (F), whichaffects the quality of the triangle wave.

Referring to FIG. 4, a circuit is provided which adds a compressor. Thespeaker output is rectified, producing +/−DC voltages proportionate tothe output level. The +/−DC voltages pass through the DC amplifiers (I,K) so that different compression ratios can be achieved. As the outputof the amplifier increases, the +/−DC amplified voltages (H, J) exceedthe voltage set by the divider resistors (R1, R2 & R8, R10). As aresult, the gain of the amplifier is reduced, thereby creating anautomatic gain control circuit that does not introduce distortion.

Referring to FIG. 5, a circuit is shown which provides a compressor inan amplifier that uses only a positive supply rail. The circuitfunctions in much the same manner as described above with respect toFIG. 3. When using only a positive supply rail, the headroom in the opamps is reduced, which may reduce the dynamic range of the compressor.This may be overcome by a slight modification and reduction of integrityof the square wave at point F. The speaker output is rectified and theDC voltages amplified. However, the insertion point changes. The outputof U1A (D) is limited by its supply voltage. The DC amplifier may supplythe voltage directly to R4. Diode D2 is added to prevent U1A fromsinking the voltage supplied from the DC amplifier. Diode D3 is added tocompensate for the voltage drop across D2. If the op amp supply voltagelimitations are a problem in the +/−power supply configuration, thensimilar modifications of the DC amplifier insertion point may be done.

Referring to FIG. 6, a circuit is used as a compressor in an amplifierthat is not a class D amplifier. The audio input is similar to theembodiment shown in FIG. 1. However, comparator outputs (C, D) do not goto switching MOSFETs. Instead, the PWM signals (PWM A, PWM B) go to adifferential amplifier (E). As a result, some of the switching frequencyis removed in a manner similar to the LC filter in FIG. 1. Additionally,the audio signal is referenced to ground instead of balanced audiooutputs. The signal then passes to a lowpass filter (F). The lowpassfilter removes the remaining switching signal. In powered subwooferapplications, the lowpass filter can be tuned to be used as part of anactive crossover network to help reduce cost. The signal next goes to apower amplifier (G). The power amplifier can be of any analog inputtopology. The audio output is then sent to the circuit, as exemplifiedin FIG. 5, and the resulting triangle wave (B) that changes based onoutput level is input to the comparators (C, D). The result is acompressor that can be used for signal processing.

Thus, various embodiments of the inventions have been described indetail above. In one embodiment, the circuit can be used to improve thepower supply rejection of a pulse width modulated digital amplifier. Insuch embodiment, the pulse width modulated digital amplifier may be,e.g., a Class D amplifier. An operational amplifier preferably bufferspower supply division references to ensure accurate low-impedancereference voltage. An oscillator provides an output having a 50% dutycycle. The oscillator is preferably a fixed oscillator. A voltage leveltranslation of the fixed oscillator provides an output in frequency tothe fixed oscillator, wherein the amplitude can vary between the setvoltage references, which may include ground. At least one integrator ispreferably provided to convert the varying amplitude square wave into atriangle wave.

In another embodiment, the circuit can be used as a compressor or as alimiter with a class D amplifier. The power supply rejection circuit asdiscussed above can be used. The audio output signal is rectified. TheDC is amplified and injected into the power supply rejection circuit atthe points described above.

In yet another embodiment, the power supply rejection circuit can beused as a compressor or as a limiter without a class D amplifier. Thepower supply rejection circuit as described above is used. The audiooutput signal is rectified. The DC is amplified and injected into thepower supply rejection circuit as discussed above at described points.The pulse width modulated signals are inputted into a differentialamplifier and a lowpass filter to restore the audio to the output.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention.

1. A method for providing improved power supply rejection in apulse-width-modulated digital device, comprising: using voltage leveltranslation to vary the amplitude of a first waveform in response tochanges in power supply voltage; converting the varied waveform into atriangle waveform; and, using the triangle waveform as an input to acomparator of the pulse-width-modulated digital device.
 2. A method forproviding improved power supply rejection in a pulse-width-modulateddigital device in accordance with claim 1, wherein said step of usingvoltage level translation to vary the amplitude of a waveform inresponse to changes in power supply voltage comprises: using a bufferingmechanism to buffer divided reference voltages; using an oscillator tooutput a first waveform, wherein the first waveform can be combined withthe divided reference voltages; and, using a circuit to output a secondwaveform having a similar frequency to the first waveform, wherein thesecond waveform has an amplitude that varies between the dividedreference voltages.
 3. The method for providing improved power supplyrejection in a pulse-width-modulated digital device in accordance withclaim 1, wherein said step of using voltage level translation to varythe amplitude of a waveform in response to changes in power supplyvoltage comprises: using a circuit that performs a voltage leveltranslation, wherein the voltage level translation outputs a secondwaveform having a similar frequency to the first waveform, wherein thesecond waveform has an amplitude that varies between divided referencevoltages.
 4. The method for providing improved power supply rejection ina pulse-width-modulated digital device in accordance with claim 1,wherein said step of converting the varied waveform into a trianglewaveform comprises: using an integrator that converts the variedwaveform into a triangle waveform that can be used with the pulse widthmodulated digital amplifier.
 5. A device for providing improved powersupply rejection of a pulse width modulated digital amplifier,comprising: a circuit that performs a voltage level translation,comprising: a buffering mechanism that buffers divided referencevoltages; an oscillator which outputs a first waveform, wherein thefirst waveform can be combined with the divided reference voltages; asubcircuit which outputs a second waveform having a similar frequency tothe first waveform, wherein the second waveform has an amplitude thatvaries between the divided reference voltages; and, an integrator thatconverts the second waveform into a triangle waveform that can be usedwith the pulse width modulated digital amplifier.
 6. The device of claim5, wherein the pulse width modulated digital amplifier is a Class Damplifier.
 7. The device of claim 5, wherein the buffering mechanism isan operational amplifier.
 8. The device of claim 5, wherein thebuffering mechanism ensures an accurate low impedance voltage.
 9. Thedevice of claim 5, wherein the second waveform is a square waveform. 10.The device of claim 5, wherein the oscillator is a fixed oscillator. 11.The device of claim 5, wherein the first waveform has a 50% duty cycle.12. The device of claim 5, wherein the divided reference voltages caninclude ground.